Composite structure with embedded optical fiber and a process for its repair

ABSTRACT

The invention relates to a composite structure ( 11 ) formed by a plurality of layers ( 13, 15, 17, 19, 21, 23 ) including an optical fiber ( 25 ) for structural monitoring purposes which is at least partly embedded in said structure ( 11 ), incorporating a protective cover ( 27 ) in those areas of its embedded part susceptible to needing repair, and to a process for repairing said embedded optical fiber comprising the following steps: identifying the optical fiber area in need of repair, removing material until reaching the cover ( 27 ), extracting said area, removing the protective cover ( 27 ), repairing the optical fiber ( 25 ), relocating the repaired area in the structure and returning the removed material.

FIELD OF THE INVENTION

The present invention relates to a composite structure incorporatingembedded optical fiber for structural monitoring purposes, and moreparticularly to a composite structure of an aircraft, as well as aprocess for its repair.

BACKGROUND OF THE INVENTION

The intensive introduction of advanced composites in primary structureshas become a fundamental process of structural optimization (based onweight savings and the improvement of mechanical properties), one of thetop priorities in the design and manufacture of a new generation ofaircrafts. The introduction of an effective structural monitoring systemcapable of predicting the failure of load paths in a structure designedaccording to damage tolerance criteria would allow optimizing its designand, accordingly, reducing its weight.

Optical fiber sensors can be effectively used to measurethermomechanical deformation and even to detect damage events operatingboth alone in passive structural monitoring systems and in combinationwith other devices, forming active monitoring systems. One of their mainadvantages is their capacity to be embedded in composite structures,being intimately integrated into the structures.

However, one of the drawbacks of this integration is the difficulty thatthe repair of these optical fiber lines has once they are embedded.Optical fiber has extremely small dimensions and is fragile in handlingwith the tools normally used in repairing composites, therefore thepossibilities of accessing it once it is embedded are very slim. This isincompatible with the reparability requirements needed when thestructural health and maintenance criteria are bound to the correctworking of the network of sensors integrated in the structure.

The present invention is aimed at solving this drawback.

SUMMARY OF THE INVENTION

In a first aspect, the present invention proposes a composite structureformed by a plurality of layers including an optical fiber forstructural monitoring purposes which is at least partly embedded in saidstructure, in which the optical fiber has a tubular protective cover inthose areas of the embedded part susceptible to requiring repair.

The incorporation of this protective cover facilitates access to theoptical fiber for its repair or maintenance, preventing itsdeterioration.

In a second aspect the present invention proposes a process of repairingthe optical fiber embedded in said composite structure comprising thefollowing steps:

Identifying the optical fiber area in need of repair.

Removing part of the layers of material until reaching the optical fiberarea in need of repair.

Extracting the optical fiber area in need of repair from the compositestructure.

Removing the protective cover.

Repairing the optical fiber.

Optionally providing a new protective cover for the repaired opticalfiber area.

Relocating the repaired optical fiber area in the composite structure.

Returning the removed parts of the layers of material.

Other features and advantages of the present invention will beunderstood from the following detailed description of an illustrativeembodiment of its object in relation to the attached figures.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic perspective view of a composite structure knownin the art formed by six layers with an optical fiber embedded betweenthe two central layers longitudinally to the reinforcement fibers and across-sectional view of area A of said structure.

FIG. 2 shows a cross-sectional view of the structure of FIG. 1 in which,using conventional composite structure repair techniques, parts of threelayers have been removed by means of sanding until reaching the embeddedoptical fiber that has been damaged during the process.

FIG. 3 shows a sectional view of a composite structure according to afirst embodiment of the present invention in which the optical fiberembedded between the two central layers of the six layers forming thestructure have a local protective cover.

FIG. 4 shows with greater detail front sectional and cross-sectionalviews of part B of FIG. 3.

FIG. 5 shows the same view as FIG. 3 after cutting the protective coverin order to reach the optical fiber and be able to repair it.

FIG. 6 shows the same view as FIG. 5 after removing the protective coverin order to reconnect the optical fiber line by joining its ends afterthe suitable maintenance operations.

FIG. 7 shows the same view as FIG. 6 after having connected, by means offusion splicing or any other standard process, the ends of the opticalfiber.

FIG. 8 shows the same view as FIG. 7 with the optical fiber repaired andthe structure reconstituted.

FIG. 9 shows the same view as FIG. 8 with the optical fiber inside aprotective cover.

FIG. 10 shows a second embodiment of the invention in a view which issimilar to that of FIG. 9, in which the embedded optical fiber isrepaired by means of an external optical fiber line.

FIG. 11 shows the same view as FIG. 10 after having connected the end ofthe embedded optical fiber with the end of the external fiber line bymeans of fusion splicing or any other standard process.

FIG. 12 shows the same view as FIG. 11 with the optical fiber repairedand the structure reconstituted.

FIG. 13 shows the same view as FIG. 12 with the embedded optical fiberinside a protective cover.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a composite structure 11 known in the art formed by aplurality of layers 13, 15, 17, 19, 21, 23 with an optical fiber 25embedded within it that can either be part of a network of sensors or itcan be a data transmission element. If for any reason it were necessaryto access said optical fiber 25 in order to repair it or for any type ofmaintenance operation, it is possible to reach it by means of compositerepair techniques consisting of the local removal of parts of thereinforcement layers 13, 15, until reaching layers 17, 19 where saidfiber is embedded. However, optical fiber 25 is a very fragile elementand operations for removing parts of the layers 13, 15, 17 can damageit, as is shown in FIG. 2; therefore by following the process known inthe art, access to the optical fiber 25 without damaging it for thepurpose of extracting it is very complex and costly.

According to the invention, the structure 11 includes a tubularprotective cover 27 which allows reaching the optical fiber 25 withoutdamaging it as is shown in FIG. 3 et seq. in order to repair it.

The possibilities of the continued existence of the optical fiber 25increase greatly by using said cover 27 locally, protecting the opticalfiber 25 formed by a core 31, a cladding 32 and a buffer coating 33.

The protective cover 27 is sized such that between it and the opticalfiber 25 there is an interface 29 that can be left open during themanufacturing process of the structure so that it can subsequently befilled with the matrix resin, or it can be closed with a filler materialsuch as silicone to prevent resin from entering the tube.

The protective cover 27 can be made of any material the toughness ofwhich allows reaching it without damaging the optical fiber locatedinside it.

The protective cover 27 can further have the secondary function offacilitating detection of the fiber 25 in the structure 11 by means ofnon-destructive inspection techniques (X-ray, ultrasound inspection orsimply a metal detector).

FIGS. 3-9 sequentially illustrate the steps comprising the process ofrepairing the optical fiber 25 embedded in a composite structure 11according to the present invention in a first embodiment thereof.

Once the protective cover 27 has been accessed, after having identifiedthe optical fiber 25 area in need of repair and having removed the partsof the layers 13, 15 and 17 necessary to do so, it will be extractedfrom the structure 11 and the cover 27 will be partially or completelyremoved (see FIGS. 5 and 6), allowing access to the fiber 25 for itsrepair or for performing the required maintenance operations byreconnecting again the ends 41, 43 previously separated by means of asplice 45 (see FIG. 7). After this, the optical fiber 25 would berelocated in its position inside the structure 11 and the removed partsof the layers 13, 15, 17 (see FIG. 8) will be returned. The repairedoptical fiber 25 can optionally be provided with a new protective cover27 (see FIG. 9).

In a second embodiment of the invention, the process would be similarexcept in that the repair of the optical fiber 25 would be performed byconnecting one of the ends 51 of the embedded optical fiber 25 to oneend 53 of an external optical fiber 55, therefore reconducting theoptical signal through a path that is an alternative to the original one(see FIGS. 10-13).

In the preferred embodiment described above any modifications comprisedwithin the scope defined by the following claims can be introduced.

1. A composite structure (11) formed by a plurality of layers (13, 15,17, 19, 21, 23) including an optical fiber (25) for structuralmonitoring purposes which is at least partly embedded in said structure(11), characterized in that said optical fiber (25) incorporates aprotective cover (27) in those areas of its embedded part susceptible toneeding repair.
 2. A composite structure (11) according to claim 1,characterized in that said cover (27) is made of a material the tenacityof which allows reaching the optical fiber (25) located inside itwithout damaging it.
 3. A composite structure (11) according to claim 2,characterized in that the space (29) comprised between the protectivecover (27) and the optical fiber (25) is occupied by a filler material.4. A composite structure (11) according to claim 2, characterized inthat the space (29) comprised between the protective cover (27) and theoptical fiber (25) is occupied by the matrix resin of the structure(11).
 5. A process of repairing the optical fiber (25) embedded in acomposite structure (11) according to claim 1, characterized in that itcomprises the following steps: a) Identifying the optical fiber (25)area in need of repair; b) Removing parts of the layers of material (13,15, 17) until reaching the protective cover (27) of the optical fiber(25) area in need of repair; c) Extracting the optical fiber (25) areain need of repair from the composite structure (11); d) Removing theprotective cover (27); e) Repairing the optical fiber (25); f)Relocating the repaired optical fiber (25) area in the compositestructure (11); g) Returning the parts of the layers of material (13,15, 17) removed in step b).
 6. A process of repairing the optical fiber(25) embedded in a composite structure (11) according to claim 5,characterized in that step e) includes the additional step of providinga new protective cover (27) for the repaired optical fiber (25) area. 7.A process of repairing the optical fiber (25) embedded in a compositestructure (11) according to claim 6, characterized in that the repairedoptical fiber area connects ends (41, 43) of optical fiber (25) embeddedin the composite structure.
 8. A process of repairing the optical fiber(25) embedded in a composite structure (11) according to claim 6,characterized in that the repaired optical fiber area connects one end(51) of optical fiber (25) embedded in the composite structure (11) withone end (53) of an optical fiber (55) located outside the compositestructure (11).
 9. A process of repairing the optical fiber (25)embedded in a composite structure (11) according to claim 2,characterized in that it comprises the following steps: a) Identifyingthe optical fiber (25) area in need of repair; b) Removing parts of thelayers of material (13, 15, 17) until reaching the protective cover (27)of the optical fiber (25) area in need of repair; c) Extracting theoptical fiber (25) area in need of repair from the composite structure(11); d) Removing the protective cover (27); e) Repairing the opticalfiber (25); f) Relocating the repaired optical fiber (25) area in thecomposite structure (11); g) Returning the parts of the layers ofmaterial (13, 15, 17) removed in step b).
 10. A process of repairing theoptical fiber (25) embedded in a composite structure (11) according toclaim 3, characterized in that it comprises the following steps: a)Identifying the optical fiber (25) area in need of repair; b) Removingparts of the layers of material (13, 15, 17) until reaching theprotective cover (27) of the optical fiber (25) area in need of repair;c) Extracting the optical fiber (25) area in need of repair from thecomposite structure (11); d) Removing the protective cover (27); e)Repairing the optical fiber (25); f) Relocating the repaired opticalfiber (25) area in the composite structure (11); g) Returning the partsof the layers of material (13, 15, 17) removed in step b).
 11. A processof repairing the optical fiber (25) embedded in a composite structure(11) according to claim 4, characterized in that it comprises thefollowing steps: a) Identifying the optical fiber (25) area in need ofrepair; b) Removing parts of the layers of material (13, 15, 17) untilreaching the protective cover (27) of the optical fiber (25) area inneed of repair; c) Extracting the optical fiber (25) area in need ofrepair from the composite structure (11); d) Removing the protectivecover (27); e) Repairing the optical fiber (25); f) Relocating therepaired optical fiber (25) area in the composite structure (11); g)Returning the parts of the layers of material (13, 15, 17) removed instep b).